An illumination method known as the relay condenser method, which is one type of Keller illumination method, is known in the prior art. This illumination method relays an image of a light source by forming an image of the light source near the pupil of a projection lens. The projection lens then forms an image of the light source (or, more accurately stated, of the relayed image of the light source) at infinity using a condenser lens to thereby make more uniform unevenness in illumination caused by unevenness in the luminosity of the light source. However, when a light source is used which has a wide variance in luminosity distribution (such as with a metal halide lamp, a xenon lamp or a halogen lamp), unevenness in illumination remains in the image of the relayed image, causing a problem.
In contrast, a method called the integrator method has been known which uses lenticular lens arrays to smooth-out unevenness in luminosity distribution properties of a light source, and various proposals have been made for using light beam dividing techniques therein, as in Japanese Laid-Open Patent Publication 3-111806.
This patent employs a concave reflector behind a light source and, in front of the light source, there are the following components: a collector lens, a first lenticular lens array, a second lenticular lens array, an image-forming lens, a field lens, and a projection lens. The first lenticular lens array comprises a plurality of lenses arranged in a two-dimensional array. The second lenticular lens array similarly is a two-dimensional array. The first lenticular lens array divides a light beam that has been emitted directly from a light source with great unevenness in brightness into divided light beams, the number of which equals the number of lenses in the first lenticular lens array. These divided light beams are then incident on, and transmitted by, the second lenticular lens array which composes the non-uniform light beams to form a more even illumination in the target illumination region. An image-forming lens and a field lens then project the combined light so as produce a more uniform beam than would exist if the light from the light source were projected without being made more uniform by the first and second lenticular lens arrays which serve as an integrator. Thus, light may be projected onto a target illumination region with less unevenness in brightness, as compared to if a single light beam were used.
In addition, in a projection-type display apparatus using two lenticular lens arrays as described above, it has been known that a plurality of light sources can be arranged having symmetry about an optical axis, with the aim of securing a stronger (i.e., more intense) illuminating light (see Japanese Laid-Open Patent Publication 6-265887).
In general, light from a light source having an intensity distribution that is not uniform is directed by a first reflector onto a first portion of a first lenticular lens array, as described above. Accordingly, particularly in the above-described case where a plurality of light sources are arranged symmetrically about an optical axis, areas with a large amount of illumination are created on a surface of the second lenticular lens array at positions separated predetermined distances from the optical axis. As the second lenticular lens array is positioned at the pupil plane of the projection lens, this results in areas of strong illumination at the pupil plane of the projection lens.
Furthermore, because the imaging quality of a lens is highest for locations on the optical axis and becomes lower with increasing distance from the optical axis, when areas with strong intensity, that is to say, areas which subjectively affect a viewed image the most, exist at positions off-axis in the pupil plane of the projection lens, the full potential of the image-formation capability of the projection lens cannot be exhibited.